Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 73

There is no guarantee where the sheathing joints will occur with respect to the collector
location. Case A of the figure shows the condition where a collector is placed at a ply-
wood joint location. If special uniform nailing is not clearly called out on the drawings,
the collector will receive one-half edge nailing and one-half field nailing because the
sheathing joints are staggered. Case B shows the collector placed at a location between
panel joints. In this case, the collector would receive only field nailing (12″ o.c.). The
result could lead to a failure of the lateral-force-resisting system.
Occasionally, the strut or collector coming off of a shear wall or frame cannot be
fully extended across the full depth of the diaphragm. In that event, an alternate load
path must be established to provide a complete load path across the diaphragm in
accordance with the code. Figure 2.41 shows three partial strut or collector configura-
tions. In configuration 1, a partial-length strut or collector is extended off of a shear
wall. Since the collector does not extend across the full depth of the diaphragm, an
alternate load path is required to be established. This is accomplished by using an adja-
cent truss that is offset to the left of the partial-length collector. The unit diaphragm
shears v , that are transferred into the collectors from the left and right diaphragms are
shown in the figure. The collective shear force in the left collector acts toward the bot-
tom of the page. That force is transferred across the transfer area into the shear wall and
partial-length in-line collector, which reacts in the opposite direction. The offset oppos-
ing forces cause a force couple that causes the transfer area to rotate in a counterclock-
wise direction. The rotation is resisted by the wall ledger that is connected to the wall at
the top of the transfer area, and by the horizontal collector at the bottom of the transfer
area that is embedded into the transfer diaphragm at the left. The horizontal collector
force is transferred out to the exterior walls through the transfer diaphragm. This com-
pletes the load paths. Configuration 2 occurs when a collector cannot be extended from
the end of a shear wall. In this case, the truss that is offset to the right of the shear wall
which extends across the full depth of the diaphragm is used as the collector. The dia-
phragm shears transferred into the collector from the left and right diaphragms are
shown in the figure. The collector force acts toward the bottom of the page and is trans-
ferred across a transfer area into the shear wall, which acts in the opposite direction.
These opposing offset forces cause the transfer area to rotate in a clockwise direction.
The rotation is resisted by the wall ledgers at the top and bottom of the transfer area.
Occasionally, drag forces are so large that one truss cannot be used as the collector. Con-
figuration 3 shows the condition where a partial-length collector extends off of the shear
wall. A collector truss is placed on each side of the partial-length collector to reduce the
forces applied to each collector truss. If the collector trusses are offset equal distances
from the partial-length collector, there the force in each collector truss will be equal. If
the trusses are offset unequal distances from the partial collector, an unbalanced moment
condition will result that will cause the transfer area to rotate in a counterclockwise
direction. The rotation is resisted by the wall ledger that is connected to the wall at the
top of the transfer area, and by the horizontal collector at the bottom of the transfer area
that is embedded into the transfer diaphragm, similar to configuration 1.
It is important to note that the transfer diaphragms and transfer areas already
receive shear from the basic diaphragm shear. The additional shear caused by the trans-
fer of the disrupted collector force must be added to the basic diaphragm shear.
The biggest challenge of transferring forces across discontinuities is the direction of
the framing and the location of the shear walls. Several framing and wall layouts are
shown in Figs. 2.42 through 2.45 which show how alternate load paths are created